Page 40 - The 'X' Chronicles Newspaper March/April 2018 Edition

40 Earth's Mysterious Hum Explained

Earth's Mysterious Hum

Explained

by Charles Q. Choi
www.livescience.com

Even planets can get a bad case of tinnitus,
according to a new study that explains Earth's
mysterious, never-ending hum.
Scientists have long known that
earthquakes can make the Earth sing like a bell
for days or months. However, in the late 1990s,
seismologists discovered the world also
constantly vibrates at very low frequencies even
when there are no quakes. This so-called
microseismic activity is too faint for humans to
feel.
Now, researchers say ocean waves are
the culprit behind these mysterious tremors.
Many scientists had previously turned to Earth's 'Hum' Helps Earth's surface. Now scientists reveal they can
also use ambient noise to image Earth's deep
ocean waves to explain our planet's unusual Probe Planet's Interior
humming. In one theory, scientists proposed the interior. The advantage of this strategy is that
vibrations were generated by huge ocean waves, "ambient noise imaging can be applied in
regions without earthquakes," Campillo said.
which can extend all the way down to the
by Charles Q. Choi The scientists installed 42 seismic
seafloor. The waves can shake the Earth as they
tumble over seafloor ridges and underwater www.livescience.com recording stations in northern Finland and
compared seismic noise signals between each
continental shelves. Another idea suggested
colliding ocean waves triggered the tremors. he global "hum" of the Earth is now helping station. By filtering out earthquake signals and
But neither idea could account for the scientists probe the planet's deep interior, a ambient seismic noise surface waves, they were
entire range of vibrations seen on earthquake group of researchers say. able to reconstruct how ambient seismic noise
sensors. The new study combines both of these Since this hum — called seismic noise, rippled through the Earth.
ideas into one model that accounts for these which is generated by sources such as storm- "Finland was a good place because it is a
microseismic signals. driven ocean waves — is detectable everywhere place with very old and homogeneous crust,"
"I think our result is an important step in on Earth, it could help scientists analyze the Campillo said. Its old age meant it had little in
the transformation of mysterious noise into an innards of the planet worldwide, investigators the way of new activity to confuse readings,
understood signal," lead study author Fabrice added in a new study detailed in the Nov. 23 while its uniform nature meant there was little
Ardhuin, an oceanographer at the French issue of the journal Science. diversity of material to complicate findings.
Research Institute for Exploitation of the Sea, Traditionally, researchers peer into the
told Live Science. interior of the Earth by analyzing seismic waves Geo toolbox
Using computer models of the ocean, generated by earthquakes. The way seismic
winds and seafloor, the scientists found that waves zip through the planet depends on Using this data, the researchers imaged the
colliding ocean waves could generate seismic physical properties of the Earth's innards, such transition zone separating the upper and lower
waves that take 13 seconds or less to complete as rock composition, temperature and pressure. layers of the Earth's mantle, the main layer just
one ripple. When it came to slower waves, they As such, the way the waves behave offers useful below Earth's crust. The top of the mantle was
found that ocean waves moving over the clues about details of Earth's geology that are about 9 miles (15 kilometers) thick and 255
seafloor could generate seismic waves with a "With these waves, seismologists miles (410 km) from the Earth's surface, while
frequency of 13 to 300 seconds. Most of the produce images in a way similar to medical its bottom was about 2.5 miles (4 km) thick and
mystery hum comes from these longer waves. imaging," researcher Michel Campillo, a 410 miles (660 km) from the Earth's surface.
The pressure of these longer ocean waves on the seismologist at Joseph Fourier University in The differences between top and bottom are due
seafloor causes most of the Earth's bell-like Grenoble, France, told OurAmazingPlanet. to changes in crystal structure resulting from
ringing, the researchers said. The problem with this strategy is that it how pressure varies according to depth.
A better understanding of this hum could depends on earthquakes. "Large earthquakes are "These changes of microstructures result
help scientists generate better maps of the rare —fortunately!" Campillo said. Quakes also in increase of seismic speeds, which we
Earth's interior, Ardhuin said. These seismic mostly recur in specific places, which leads to eventually detect when waves are reflected on
waves penetrate deep into the planet's mantle some areas being imaged well but leaving others the layers where they occur," Campillo said.
and potentially all the way to Earth's core. This relatively obscure. Ultimately, ambient seismic noise might
means that analyzing these waves could help In addition to seismic waves from not only help researchers scan the mantle
yield a more detailed picture of the planet's earthquakes, the interior of the Earth is pervaded transition zone — where the upper and lower
structure, he said. by seismic noise, a collective hum resulting layers meet — but also probe all the way down
The researchers added there could be from the bombardment of Earth's surface by a to the core-mantle boundary.
still more sources of microseismic activity. For variety of sources, such as the swelling of "Ambient noise is another element in the
instance, perhaps ocean waves traveling along oceans during storms. geophysicist's toolbox," Campillo said. "Our
shorelines or traveling down underwater "The noise was regarded as useless and study suggests that it could be developed
mountains and mid-ocean ridges can generate even problematic since it hides slight earthquake everywhere, allowing for new collections of
seismic waves, Ardhuin said. signals," Campillo said. observations at the global scale."otherwise
Ardhuin and his colleagues Lucia largely hidden from view.[]
Gualtieri and Eléonore Stutzmann at the Paris Tracking noise
Institute of Earth Physics detailed their findings
in the Feb. 16 issue of the journal Geophysical However, in recent years, by analyzing large
Research Letters. [] amounts of seismic data collected over time,
investigators successfully followed ambient
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seismic noise waves as they rippled across
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